U.S. patent number 10,219,090 [Application Number 13/779,314] was granted by the patent office on 2019-02-26 for method and detector of loudspeaker diaphragm excursion.
This patent grant is currently assigned to Analog Devices Global. The grantee listed for this patent is Analog Devices Global. Invention is credited to Robert Adams, Kim Spetzler Berthelsen.
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United States Patent |
10,219,090 |
Adams , et al. |
February 26, 2019 |
Method and detector of loudspeaker diaphragm excursion
Abstract
The present invention relates in one aspect to a method of
detecting diaphragm excursion of an electrodynamic loudspeaker. The
method comprises steps of generating an audio signal for
application to a voice coil of the electrodynamic loudspeaker and
adding a high-frequency probe signal to the audio signal to
generate a composite drive signal. The method further comprises a
step of applying the composite drive signal to the voice coil
through an output amplifier and detecting a modulation level of a
probe signal current flowing through the voice coil.
Inventors: |
Adams; Robert (Acton, MA),
Berthelsen; Kim Spetzler (Koge, DK) |
Applicant: |
Name |
City |
State |
Country |
Type |
Analog Devices Global |
Hamilton |
N/A |
BM |
|
|
Assignee: |
Analog Devices Global
(Hamilton, BM)
|
Family
ID: |
50156642 |
Appl.
No.: |
13/779,314 |
Filed: |
February 27, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140241536 A1 |
Aug 28, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04R
3/007 (20130101); H04R 29/003 (20130101) |
Current International
Class: |
H04R
29/00 (20060101); H04R 3/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102802104 |
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Nov 2012 |
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CN |
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19804992 |
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Aug 1999 |
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DE |
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2453670 |
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May 2012 |
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EP |
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2498511 |
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Sep 2012 |
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EP |
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2538699 |
|
Dec 2012 |
|
EP |
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97/03536 |
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Jan 1997 |
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WO |
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Other References
Dodd, Mark, et al: Voice Coil Impedance as a Function of Frequency
and Displacement. cited by applicant .
Klippel, Wolfgang, et al: Loudspeaker Noninearities--Causes,
Parameters, Symptoms. Dresden, Germany. cited by applicant .
Thorborg, Knud, et al: An improved Electrical Equivalent Circuit
Model for Dynamic Moving Coil Transducers. Audio Engineering
Society, Convention Paper, presented at the 122nd Convention May
5-8, 2007, Vienna, Austria. cited by applicant .
Extended European Search Report dated Jul. 22, 2014, in counterpart
European application No. 14156524.2. cited by applicant .
Clark, "Amplitude Modulation Method for Measuring Linear Excursion
of Loudspeakers", Audio Engineering Society Convention Papers, AES
Convention 89, Sep. 1, 1990, 18 pages. cited by applicant .
Klippel, "Assesment of Voice-Coil Peak Displacement Xmax", Journal
of the Audio Engineering Society, vol. 51, No. 5, May 2003, pp.
307-323. cited by applicant .
Klippel, "Tutorial: Loudspeaker Nonlinearities--Causes, Parameters,
Symptoms", Journal of the Audio Engineering Society, vol. 54, No.
10, Oct. 2006, pp. 907-939. cited by applicant .
Clark et al., "Modeling and Controlling Excursion-Related
Distortion in Loudspeakers", Audio Engineering Society Convention
Papers, AES Convention 106, May 1999, 14 pages. cited by applicant
.
Klippel, "Active Reduction of Nonlinear Loudspeaker Distortion",
Active 99 Conference, Dec. 1999, 12 pages. cited by applicant .
Luo et al., "A Model Based Excursion Protection Algorithm for
Loudspeakers", IEEE International Conference on Acoustics, Speech
and Signal Processing (ICASSP), Mar. 2012, pp. 233-236. cited by
applicant .
"European Application Serial No. 14156524.2, Office Action dated
Mar. 24, 2016", 6 pgs. cited by applicant .
"European Application Serial No. 14156524.2, Office Action dated
Jun. 30, 2015", 6 pgs. cited by applicant .
"European Application Serial No, 14156524,2, Response filed Mar. 2,
2015 to European Search Report dated Jul. 22, 2014", 13 pgs. cited
by applicant .
"European Application Serial No. 14156524.2, Response filed Dec.
19, 2015 to Office Action dated Jun. 3, 2015", 25 pgs. cited by
applicant .
"Chinese Application Serial No. 201410065381.1, Office Action dated
Nov. 14, 2016", (w/ English Translation), 18 pgs. cited by
applicant .
"European Application Serial No. 14156524.2, Response filed Mar. 2,
2015 to Extended European Search Report dated Jul. 22, 2014", 18
pgs. cited by applicant .
"European Application Serial No. 14156524.2, Response filed Jul. 4,
2016 to Office Action dated Mar. 24, 2016", 8 pgs. cited by
applicant.
|
Primary Examiner: Nguyen; Duc
Assistant Examiner: Mohammed; Assad
Attorney, Agent or Firm: Schwegman Lundberg & Woessner,
P.A.
Claims
The invention claimed is:
1. A method of detecting diaphragm excursion of an electrodynamic
loudspeaker, comprising steps of: receiving a digital audio signal
having a first sample rate, using an up-sampler and modulator
circuit, up-sampling the digital audio signal to a greater second
sample rate and adding a high-frequency probe signal to the
up-sampled digital audio signal to generate a pulse-modulated
composite drive signal, wherein the probe signal has a frequency
that exceeds a Nyquist frequency of the received digital audio
signal, applying the pulse-modulated composite drive signal to a
voice coil of the electrodynamic speaker through an output
amplifier, detecting a composite drive signal current flowing
through the voice coil in response to the application of the
pulse-modulated composite drive signal, detecting a modulation
level of a probe signal current from the composite drive signal
current, and identifying an excursion of a diaphragm of the
loudspeaker based on the detected modulation level of the probe
signal current.
2. The method of claim 1, wherein the detecting the modulation
level of the probe signal current-comprises steps of: band-pass
filtering the composite drive signal current to attenuate audio
signal components therein, and detecting the modulation level of
the probe signal current from the band-pass filtered composite
drive signal current.
3. The method of claim 1, wherein the detecting the modulation
level of the probe signal current comprises: detecting an envelope
of the probe signal current.
4. The method of claim 2, wherein the detecting the modulation
level of the probe signal current comprises: rectifying and lowpass
filtering the band-pass filtered composite drive signal
current.
5. The method of claim 1, comprising a step of: one of pulse
density modulating and pulse width modulating the audio signal in
the output amplifier to supply a PDM or PWM modulated composite
drive signal to the voice coil of the electro dynamic
loudspeaker.
6. The method of claim 1, comprising a step of: up-sampling the
digital audio signal by one or more intermediate up-sampling stages
producing digital audio signals at respective intermediate sample
rates in-between the first and the second sample rates.
7. The method of claim 6, comprising steps of: generating the
high-frequency probe signal as a digital high-frequency probe
signal, and adding the digital high-frequency probe signal to one
of the digital audio signals at the intermediate sample rates or to
a final digital audio signal to generate the composite drive signal
in digital format.
8. The method of claim 7, wherein the high-frequency digital probe
signal is added to the up-sampled digital audio signal at an
intermediate sample rate that is at least two times higher than a
frequency of the digital high-frequency probe signal.
9. The method of claim 1, comprising a step of: comparing the
detected modulation level of the probe signal current with a
specified modulation level threshold.
10. The method of claim 9, comprising a step of: attenuating a
level of the digital audio signal if the detected modulation level
of the probe signal current matches the modulation level
threshold.
11. The method of claim 1, wherein the high-frequency probe signal
comprises a sine wave with a frequency above 10 kHz.
12. The method of claim 1, comprising a step of: adding the
high-frequency probe signal to the audio signal by modulating the
audio signal with a predetermined carrier frequency such that the
high-frequency probe signal is produced by carrier frequency
components.
13. The method of claim 1, comprising steps of: detecting a level
of the audio signal, comparing the level of the audio signal with a
predetermined threshold level, and adding the high-frequency probe
signal to the audio signal exclusively when the level of the audio
signal exceeds the predetermined threshold level.
14. The method of claim 1, comprising steps of: determining an
excursion limit of the electrodynamic loudspeaker during a
calibration measurement on the electrodynamic loudspeaker or an
electrodynamic loudspeaker of the same type, determining and
recording the modulation level of the probe signal current
corresponding to the excursion limit of the loudspeaker, and
deriving the pre-set modulation level criteria from the recorded
modulation level of the probe signal current at the excursion
limit.
15. The method of d claim 1, comprising a step of: sampling the
probe signal current by an A/D converter to provide a sampled or
digital probe signal current.
16. A loudspeaker excursion detector for electrodynamic
loudspeakers, comprising: an audio signal input for receipt of a
digital audio signal supplied by an audio signal source, a probe
signal source for generation of a high-frequency digital probe
signal, an up-sampler and modulator circuit configured to up-sample
the audio signal and then combine the up-sampled audio signal with
the probe signal to provide a composite drive signal, an output
amplifier configured to supply the composite drive signal at a pair
of output terminals connectable to a voice coil of an
electrodynamic loudspeaker, a current detector configured for
detecting from the voice coil a composite drive signal current
flowing through the voice coil in response to the application of
the composite drive signal, and a modulation detector configured to
determine a modulation level of a probe signal current of the
composite drive signal current, wherein the modulation level
indicates an excursion characteristic of a diaphragm of the
electrodynamic loudspeaker.
17. The loudspeaker excursion detector of claim 16, comprising: a
band-pass filter coupled for receipt of the composite drive signal
current and providing the probe signal current at a filter
output.
18. The loudspeaker excursion detector of claim 17, wherein the
modulation detector comprises an envelope detector coupled to the
output of the band-pass filter to detect the modulation level.
19. The loudspeaker excursion detector of claim 16, wherein the
output amplifier comprises a class D power stage configured to
supply a pulse modulated composite drive signal to the voice coil
of the electrodynamic loudspeaker.
20. The loudspeaker excursion detector of claim 19, wherein the
up-sampler and modulator circuit is configured to up-sample the
audio signal through one or more intermediate up-sampling stages
configured to produce one or more digital audio signal(s) at
respective intermediate sample rate(s) in-between an initial audio
signal sample rate and a final sample rate.
21. The loudspeaker excursion detector according to of claim 20,
wherein the probe signal source is configured to generate the
high-frequency probe signal at a probe signal sample rate; and
wherein the up-sampler and modulator circuit comprises a digital
signal combiner configured to add the high-frequency probe signal
to the digital audio signal at an intermediate sample rate at least
two times higher than a frequency of the high-frequency probe
signal.
22. The loudspeaker excursion detector of claim 20, wherein the
output amplifier comprises one of a pulse density modulated and
pulse width modulated power stage coupled for receipt of the
digital audio signal at the final sample rate.
23. The loudspeaker excursion detector of claim 16, comprising: a
comparator configured for comparing the detected modulation level
of the probe signal current with a pre-set modulation level
criteria.
24. The loudspeaker excursion detector of claim 23, comprising: a
diaphragm excursion limiter configured to attenuate a level of the
audio signal if the detected modulation level of the probe signal
current matches the pre-set modulation level criteria.
25. The loudspeaker excursion detector of claim 16, wherein the
current detector comprises an analog-to-digital (A/D) converter to
provide a sampled or digital signal representative of the composite
drive signal current.
26. The loudspeaker excursion detector of claim 16, wherein the
output amplifier comprises a predetermined output impedance less
than 1.0.OMEGA. at the probe signal frequency.
27. A semiconductor substrate having the loudspeaker excursion
detector of claim 16 integrated thereon.
28. An excursion control system for electrodynamic loudspeakers,
comprising: an electrodynamic loudspeaker comprising a movable
diaphragm assembly for generating audible sound in response to
actuation of the assembly, the loudspeaker excursion detector of
claim 16 electrically coupled to the movable diaphragm assembly, an
audio signal source operatively coupled to the audio signal input
of the loudspeaker excursion detector.
29. The excursion control system for electrodynamic loudspeakers of
claim 28, wherein the audio signal source comprises a DSP
delivering a digital audio signal to the loudspeaker excursion
detector.
Description
The present invention relates in one aspect to a method of
detecting diaphragm excursion of an electrodynamic loudspeaker. The
method comprises steps of generating an audio signal for
application to a voice coil of the electrodynamic loudspeaker and
adding a high-frequency probe signal to the audio signal to
generate a composite drive signal. The method further comprises a
step of applying the composite drive signal to the voice coil
through an output amplifier and detecting a modulation level of a
probe signal current flowing through the voice coil.
BACKGROUND OF THE INVENTION
The present invention relates to a method of detecting diaphragm
excursion or displacement of electrodynamic loudspeakers and a
corresponding loudspeaker excursion detector. Methodologies and
devices for detecting diaphragm excursion of electrodynamic
loudspeakers are highly useful for numerous purposes for example in
connection with diaphragm excursion control or limitation.
Diaphragm excursion control is useful to prevent the diaphragm and
voice coil assembly being driven beyond its maximum allowable peak
excursion. Unless proper precautionary measures are taken, powerful
amplifiers may force such high levels of drive currents into the
voice coil that the diaphragm and voice coil assembly is driven
beyond its maximum allowable peak excursion leading to various
kinds of mechanical damage. Hence, there is a need to
monitor/detect the instantaneous displacement of a loudspeaker
diaphragm to prevent mechanical damage caused by excursions
exceeding the excursion limit of the type of electrodynamic
loudspeaker in question. This issue is of significant importance in
numerous areas of loudspeaker technology such as high power
loudspeakers for public address systems, automotive speaker and
home Hi-Fi applications as well as miniature loudspeakers for
portable communication devices such as smartphones, laptop
computers etc.
Many attempts have been made in the prior art to detect or estimate
instantaneous displacement of loudspeaker diaphragms for the above
outlined purposes. These attempts have often been based on complex
non-linear models of the particular loudspeaker type in question.
Model-based approaches require careful analysis of the
electro-mechanical and magnetic characteristics of the particular
loudspeaker type of interest. Likewise, model based approaches
require complex real-time computations on the non-linear
loudspeaker model to estimate the actual excursion of the real
operative loudspeaker. Complex computations leads to high power
consumption of a Digital Signal Processor executing the model based
estimate and/or control algorithm which is particularly undesired
for battery powered communication devices like smartphones etc. The
model parameters can furthermore be difficult to determine
accurately and may vary over temperature, time and between
individual loudspeaker samples of the same type. Other attempts
have been based on transducer signals supplied by various types of
acceleration and velocity sensors attached to the diaphragm or
voice coil.
Hence, it is of significant interest and value to provide a
relatively simple method for estimating or detecting the
displacement or excursion of the loudspeaker diaphragm without
relying on complex non-linear models of the particular loudspeaker
type. The displacement detection may be accompanied by a suitable
mechanism for limiting the diaphragm displacement if it exceeds the
loudspeaker's maximum allowable peak excursion. The diaphragm
excursion detection mechanism and the corresponding detector should
preferably be operative with minimal, or without, a priori
knowledge of linear and non-linear properties of the loudspeaker to
simplify or entirely eliminate calibration procedures.
EP 2 453 670 A1 discloses a method to generate a control signal
that can be used for mechanical loudspeaker protection or for other
signal pre-processing functions in a loudspeaker control system
without requiring knowledge of the mechanical parameters of the
loudspeaker. The control signal may be a measure of how close the
loudspeaker is driven to its mechanical displacement limit and is
based on a so-called arbitrarily scaled frequency dependent input
voltage to excursion transfer function. The latter transfer
function is derived during a calibration procedure from a plurality
of drive voltage and current measurements on the loudspeaker at
different audio frequencies.
U.S. 2009/268918 A1 discloses mechanical protection of loudspeakers
using digital processing and predictive estimation of instantaneous
displacement of the voice coil in a loudspeaker transducer. The
invention solves the problem of limiting the voice coil
displacement of the transducer by applying a look-a-head based
linear or non-linear predictor and a controller operating directly
on the displacement signal in order to finally convert back into
the incoming signal domain.
U.S. Pat. No. 5,931,221 B1 discloses with reference to FIG. 7, a
dynamic loudspeaker driving apparatus which comprises a power
amplifier coupled to an electrodynamic loudspeaker and a feedback
circuit for providing improved motional feedback. The feedback
circuit negatively feedbacks the detected motional voltage to the
power amplifier. A bridge circuit is used to extract a motional
voltage produced by the loudspeaker. A leg of the bridge includes
an impedance which corresponds to the impedance of the dynamic
loudspeaker including its motional impedance so to provide a more
accurate motional feedback voltage.
SUMMARY OF THE INVENTION
A first aspect of the invention relates to a method of detecting
diaphragm excursion an electrodynamic loudspeaker, comprising steps
of:
generating an audio signal for application to a voice coil of the
electrodynamic loudspeaker, adding a high-frequency probe signal to
the audio signal to generate a composite drive signal, applying the
composite drive signal to the voice coil through an output
amplifier, detecting a modulation level of a probe signal current
flowing through the voice coil.
The skilled person will appreciate that each of the audio signal,
high-frequency probe signal, the composite drive signal and the
probe signal current may be represented by an analog signal for
example as a voltage, current, charge etc. or alternatively be
represented by a digital signal, e.g. coded in binary format at a
suitable sample rate and resolution.
The present invention provides in one aspect a method of detecting
the excursion or displacement of a diaphragm of the electrodynamic
loudspeaker which method exploits the excursion dependent change of
voice coil inductance of an electrodynamic loudspeaker. This
excursion-dependent inductance of the voice coil is reflected in a
corresponding excursion-dependent change of the high-frequency
impedance of the voice coil of the electrodynamic loudspeaker. This
change of high-frequency impedance can be detected during real-time
operation of the electrodynamic loudspeaker by adding a preferably
inaudible high-frequency probe or pilot signal to the audio signal
and detecting the level of modulation of the probe signal current
flowing through the voice coil as a result of the high-frequency
probe signal component of the composite drive signal applied to the
voice coil. The composite drive signal is preferably applied to the
voice coil through a suitable output or power amplifier. By
detecting the modulation level of the probe signal current, the
excursion of the electrodynamic loudspeaker is detectable.
The mechanism behind the excursion-dependent inductance and
high-frequency impedance of the voice coil of electrodynamic
loudspeakers is discussed in detail below in connection with FIGS.
2 & 3.
The audio signal may comprise speech and/or music supplied from a
suitable audio source such as radio, CD player, network player, MP3
player. The audio source may also comprise a microphone generating
a real-time microphone signal in response to incoming sound.
The skilled person will understand that the selected frequency of
the high-frequency probe signal can vary considerably dependent on
impedance characteristics of a specific electrodynamic loudspeaker
and various other application constraints. In one exemplary
embodiment, the high-frequency probe signal comprises a sine wave
with a frequency above 10 kHz, more preferably above 20 kHz. The
frequency of the high-frequency probe signal is preferably
sufficiently high to be inaudible to the listener or user. The
inaudible character of the high-frequency probe signal may either
be caused by the probe frequency being above the audible limit of
human hearing (i.e. above about 20 kHz) or because the loudspeaker
is incapable of reproducing noticeable sound pressure at the probe
signal frequency. The frequency of the high-frequency probe signal
may accordingly vary considerably; A large diameter woofer may be
incapable of producing noticeable sound response above for example
1 kHz such that the high-frequency probe signal may be placed at,
or slightly above, 1 kHz for this type of loudspeaker. A small
diameter full-range miniature electrodynamic loudspeaker for
portable communication devices or music players may on the other
hand produce useful sound pressure up to 15 kHz or even 20 kHz such
that the high-frequency probe signal preferably is placed at, or
slightly above, 20 kHz for this type of loudspeaker to remain
inaudible in all situations. Furthermore, the high-frequency probe
signal is preferably also located at a frequency range where the
voice coil impedance of the loudspeaker exhibits a pronounced
inductive behaviour. This is advantageous for level detection
accuracy because of the higher modulation of the probe signal
current at frequencies where the non-linear voice-coil inductance
provides a significant contribution to the total voice-coil
impedance.
The skilled person will appreciate that the actual detection of the
modulation level of the probe signal current may be accomplished in
various ways in either the analog or digital domain. In a preferred
embodiment, the detection of the modulation level of the probe
signal current comprises steps of:
detecting a composite drive signal current flowing through the
voice coil in response to the composite drive signal,
band-pass filtering the composite drive signal current to attenuate
audio signal components therein,
detecting the modulation level of the probe signal current from the
band-pass filtered composite drive signal current.
The band-pass filtering of the composite drive signal current may
be achieved by band-pass filtering a suitable voltage, current,
charge etc. signal proportional to the voice-coil current to
produce the probe signal current dependent on the selected voice
coil current detection mechanism. The band-pass filtering removes
audio signal components from the composite drive signal current and
passes substantially only the probe-signal components. Thereafter,
the, modulation level of the probe signal current may be detected
by extracting an envelope of the composite drive signal current
using conventional methods such peak or average detection, and
finally detecting modulation of the envelope signal of the probe
signal current.
The frequency selective filtering of the composite voice-coil
current is preferably adapted to suppress all other frequency
components than those proximate to high-frequency probe signal.
Large amplitude low frequency components of the audio signal, which
tend to determine the excursion of the loudspeaker diaphragm,
appear as AM side-bands close to the probe signal frequency and
therefore remain largely unattenuated by the frequency selective
filtering. Hence, the envelope waveform of the band-pass filtered
composite drive signal current reflects the excursion of the
diaphragm. Consequently, one embodiment of the present methodology
relies on detecting the envelope of the band-pass filtered probe
signal current to detect the modulation level. This envelope may be
detected by various mechanisms such as traditional AM demodulation
techniques. The latter include rectification and low-pass filtering
of the band-pass filtered composite drive signal current. In other
embodiments, the modulation level of the filtered probe signal
current may be detected or estimated by applying suitable bottom
and top trackers to the envelope waveform of a digitally converted
filtered probe signal current.
The composite drive signal supplied to the voice coil of the
electrodynamic loudspeaker may advantageously be pulse modulated to
take advantage of the high power-conversion efficiency of pulse
modulated amplifiers. This pulse modulation may be accomplished by
utilizing a switching type or class D type of output amplifier
topology for example PDM or PWM output amplifiers. The latter types
of class D amplifiers provide pulse density or pulse width
modulation of the audio signal to generate the composite drive
signal in modulated format. In the alternative, the output
amplifier may comprise traditional non-switched power amplifier
topologies like class A or class AB. An output impedance of the
power amplifier is preferably smaller than the voice coil impedance
of the intended or target loudspeaker(s) throughout the relevant
audio frequency range, e.g. 20 Hz to 20 kHz. Hence, the skilled
person will appreciate that the output impedance of the output
amplifier may vary significantly depending upon impedance
characteristics of the target electrodynamic loudspeaker(s) in
question. In a number of useful embodiments of the invention, the
output impedance of the output amplifier is smaller than
1.0.OMEGA., such as smaller than 0.5.OMEGA. or 0.1.OMEGA.
throughout the relevant audio frequency range. These ranges of
relatively small output impedances minimize power dissipation in
output devices/transistors of the output amplifier, in particular
when coupled to low-impedance electrodynamic loudspeakers, e.g.
loudspeakers with nominal impedance in a range between 2 and 8
ohms. The output impedance of the output amplifier is preferably
also smaller than 1.0.OMEGA., such as smaller than 0.5.OMEGA., or
0.1.OMEGA., at the frequency of the probe signal.
In numerous useful embodiments of the present methodology, the
audio signal may be generated in digital format as a first digital
audio signal at a first sample rate. The first sample rate is
preferably relatively low such as below 44.1 kHz or below 32 kHz to
reduce power consumption of associated digital processing equipment
and circuits. However, the use of the above-mentioned class D type
of output amplifier topology requires a much higher sampling
frequency than first sample rate to provide efficient conversion.
Hence, the methodology preferably comprises generating the audio
signal as the first digital audio signal at the first sample rate,
up-sampling the first digital audio signal to generate a final
digital audio signal at a final sample rate higher than the first
sample rate. Finally, the final digital audio signal is preferably
either pulse density modulated or pulse width modulated in the
output amplifier. The final sample rate may be between 4 and 32
times higher than the first sample rate.
The up-sampling of the first digital audio signal to final digital
audio signal is preferably performed by one or more intermediate
up-sampling stages producing digital audio signals at respective
intermediate sample rates in-between the first and the final sample
rate.
According to a preferred embodiment of the present methodology, the
high-frequency probe signal is generated in digital format as a
digital high-frequency probe signal and added to one of the digital
audio signals at the intermediate sample rates or to the final
digital audio signal to generate a composite drive signal in
digital format. In a particularly advantageous variant of the
latter embodiment, the high-frequency digital probe signal is added
to a digital audio signal with intermediate sample rate at least
two times higher than a frequency of the digital high-frequency
probe signal. The up-sampling the first digital audio signal to the
intermediate sample rate digital audio signal above the Nyquist
frequency of the digital high-frequency probe signal before
addition of the digital high-frequency probe signal is beneficial
in numerous applications. This up-sampling operation allows an
audio signal generator supplying the first digital audio signal to
operate with a relatively low sampling frequency or rate e.g. 32
kHz despite the use of a relatively high frequency of the digital
probe signal such as 40 kHz situated far above the Nyquist
frequency of the first digital audio signal. The relatively low
sampling frequency of the audio signal generator reduces its power
consumption. The up-sampling of the first digital audio signal may
for example be accomplished in the above-mentioned modulator
portion of the class D amplifier without the expense of additional
digital processing hardware and its associated power consumption.
The skilled person will appreciate that various types of signal
quantisation and noise shaping may be applied to the final digital
audio signal and/or to the intermediate digital audio signals in a
modulator portion of the class D amplifier.
The present methodology of detecting diaphragm excursion may be
configured to limit or control the diaphragm excursion to prevent
various kinds of mechanical damage to the loudspeaker. The
mechanical damage may be caused by collision between movable
loudspeaker components such as the voice coil, diaphragm or voice
coil former and stationary components such as the magnetic circuit.
In one such embodiment of the present methodology the latter
comprises steps of:
comparing the detected modulation level of the probe signal current
with a pre-set modulation level criteria such as a modulation level
threshold.
This excursion control may be accomplished by a variety of
mechanisms for example by attenuating a level of the audio signal
if the detected modulation level of the probe signal current
matches the pre-set modulation level criteria such as exceeding the
modulation level threshold. The attenuation of the audio signal
level may be accomplished by selectively attenuating low-frequency
components of the digital audio signal, as the latter are more
likely to drive the loudspeaker above its excursion limit, or broad
band attenuating the entire audio spectrum of the digital audio
signal.
The modulation level criteria or threshold may have been determined
in numerous ways for example through a previous calibration
measurement on the loudspeaker in question. A preferred embodiment
of the present methodology comprises steps of:
determining an excursion limit of the electrodynamic loudspeaker
during a calibration measurement on the electrodynamic loudspeaker
or an electrodynamic loudspeaker of the same type,
determining and recording the modulation level of the probe signal
current corresponding to the excursion limit of the
loudspeaker,
deriving the pre-set modulation level criteria from the recorded
modulation level of the probe signal current at the excursion
limit.
The pre-set modulation level criteria may be stored in digital
format in a suitable data memory location of a loudspeaker
excursion detector implementing the present diaphragm excursion
detection. Alternatively, the pre-set modulation level criteria may
be stored in data memory of a signal processor, such as a
microprocessor or DSP operatively coupled to the loudspeaker
excursion detector as described below in additional detail.
In one embodiment, the high-frequency probe signal is added to the
audio signal as an integral operation of a pulse modulation of the
audio signal in a class D output amplifier. Hence, the
high-frequency probe signal may be added to the audio signal by
modulating the audio signal with a predetermined carrier frequency
in a pulse modulated output amplifier such that the high-frequency
probe signal is produced by carrier frequency components. The
high-frequency probe signal therefore comprises the carrier
frequency component of the pulse modulation. This type of carrier
frequency components are inherently added to the drive signal
supplied to the loudspeaker by class D output amplifiers despite
certain output filters which may attenuate the level of these
carrier frequency components. While this carrier frequency
component is unwanted under many circumstances, this particular
embodiment exploits the presence of the carrier frequency component
to eliminate separate high-frequency probe signal generation.
Hence, a separate digital or analog probe signal generator and
corresponding signal combiner are both saved leading to a reduction
of the complexity of the present loudspeaker excursion detector and
corresponding methodology.
The addition of the high-frequency probe signal to the audio signal
may be performed substantially continuously during operation of the
diaphragm excursion detection methodology or discontinuously for
example solely during time periods where certain characteristics of
the audio signal are met. According to a preferred embodiment, the
methodology comprises steps of:
comparing the level of the audio signal with a predetermined
threshold level,
adding the high-frequency probe signal to the audio signal
exclusively when the level of the audio signal exceeds the
predetermined threshold level.
Hence, when the level of the audio signal falls below the
predetermined threshold level the addition of the high-frequency
probe signal may be interrupted. In this embodiment, the
predetermined threshold level ensures the high-frequency probe
signal is added only to the audio signal under conditions where the
audio signal has sufficient level or amplitude to force the
loudspeaker diaphragm close to, or above, its excursion limit. The
interruption of the high-frequency probe signal may serve to
minimise possible audible artifacts associated with the
high-frequency probe signal, in particular if the high-frequency
probe signal is placed in the audible frequency range. In the
alternative, the level of the high-frequency probe signal may be
attenuated with a certain factor e.g. 20 dB or more when the level
of the audio signal falls below the predetermined threshold
level.
As previously mentioned, the present methodology may advantageously
be performed at least partly in the digital domain. In one
embodiment, the probe signal current is sampled by an A/D converter
to provide a sampled or digital probe signal current. The presence
of the probe signal current in the digital domain is of course
particularly well-suited for detection of the modulation level by a
DSP algorithm or application executing on the previously discussed
signal processor. The skilled person will appreciate that the probe
signal current may be represented by any suitable voltage, current
or charge signal proportional thereto.
A second aspect of the invention relates to a loudspeaker excursion
detector for electrodynamic loudspeakers, comprising:
an audio signal input for receipt of an audio signal supplied by an
audio signal source,
a probe signal source for generation of a high-frequency probe
signal,
a signal combiner configured to combine the audio signal with the
high-frequency probe signal to provide a composite drive
signal,
an output amplifier configured to supply the composite drive signal
at a pair of output terminals connectable to a voice coil of an
electrodynamic loudspeaker,
a current detector configured for detecting a composite drive
signal current flowing through the voice coil in response to the
application of the composite drive signal,
a modulation detector configured to determine a modulation level of
a probe signal current of the composite drive signal current.
The properties of the output amplifier have been disclosed in
detail above in connection with the corresponding excursion
detection methodology. The Class D output amplifier may comprises a
half-bridge driver stage with a single output coupled to the
electrodynamic loudspeaker or a full-bridge/H-bridge driver stage
with the pair of output terminals coupled to respective sides or
terminals of the electrodynamic loudspeaker.
The skilled person will appreciate that the current detector may
comprise various types of current sensors for example a current
mirror connected to an output transistor of the output amplifier or
a small sense resistor coupled in series with the loudspeaker voice
coil. The composite drive signal current may accordingly be
represented by a proportional/scaled sense voltage. The latter
voltage may be sampled by the previously discussed A/D converter to
allow processing and modulation detection of the probe signal
current in the digital domain. The loudspeaker excursion detector
preferably comprises a band-pass filter coupled for receipt of the
composite drive signal current and providing the probe signal
current at a filter output as discussed in detail above in
connection with the corresponding feature of the excursion
detection methodology.
A preferred embodiment of the modulation detector comprises an
envelope detector coupled to the output of one of a band-pass
filter to detect the modulation level of the probe signal current.
The envelope detector may comprise an AM demodulator and operate
either in the digital domain or analog domain as discussed in
detail above in connection with the corresponding feature of the
excursion detection methodology.
The loudspeaker excursion detector may comprise a diaphragm
excursion limiter to control and/or limit diaphragm excursion to
prevent mechanical damage as discussed in detail above in
connection with the corresponding feature of the excursion
detection methodology. The diaphragm excursion limiter may comprise
a comparator configured for comparing the detected modulation level
of the probe signal current with a pre-set modulation level
criteria such as a modulation level threshold for the previously
discussed reasons. The diaphragm excursion limiter is preferably
configured to attenuate the level of the audio signal if the
detected modulation level of the probe signal current matches the
pre-set modulation level criteria--for example exceeds the
modulation level threshold.
The audio signal source and the probe signal source may be
configured to supply the audio signal and the high-frequency probe
signal, respectively, in digital format to provide a digital
composite drive signal at a first sample rate to an input of the
pulse density modulated or pulse width modulated power stage.
According to a preferred embodiment, the output amplifier comprises
a digital up-sampling circuit configured for receipt and
up-sampling the first digital audio signal to a final digital audio
signal at a final sample rate, higher than the first sample rate,
to generate a digital composite drive signal. The digital
up-sampling circuit comprises one or more intermediate up-sampling
stages configured to produce one or more digital audio signal(s) at
respective intermediate sample rate(s) in-between the first sample
rate and the final sample rate. In a particularly advantageous
embodiment of the present loudspeaker excursion detector the probe
signal source is configured to generating the high-frequency probe
signal as a digital high-frequency probe signal and the digital
up-sampling circuit comprises a digital signal combiner configured
to add the digital high-frequency probe signal to a digital audio
signal at an intermediate sample rate at least two times higher
than a frequency of the digital high-frequency probe signal. The
advantages offered by this embodiment of the invention have
previously been described in detail in connection with the first
aspect of the invention.
The final digital audio signal may be applied directly or
indirectly to an input of the previously discussed pulse modulated
output amplifier e.g. a class D amplifier.
A third aspect of the invention relates to a semiconductor
substrate or die having an loudspeaker excursion detector according
to any of the above-described embodiments integrated thereon. The
semiconductor substrate may be fabricated in a suitable CMOS or
DMOS semiconductor process.
A fourth aspect of the invention relates to an excursion control
system for electrodynamic loudspeaker. The excursion control system
comprising:
an electrodynamic loudspeaker comprising a movable diaphragm
assembly for generating audible sound in response to actuation of
the assembly,
a loudspeaker excursion detector, according to according to any of
the above-described embodiments thereof, electrically coupled to
the movable diaphragm assembly.
The excursion control system furthermore comprises an audio signal
source which is operatively coupled to the audio signal input of
the loudspeaker excursion detector. The audio signal source may
comprise a programmable or hard-wired Digital Signal Processor
(DSP) operating inter alia as a digital audio signal source for the
present loudspeaker excursion detector. The digital audio signal
supplied by the programmable or hard-wired DSP may be generated by
the DSP itself or retrieved from an audio file stored in a readable
data memory coupled to the excursion control system. The digital
audio signal may comprise a real-time digital audio signal supplied
to a DSP audio input from an external digital audio source such as
a digital microphone. The real-time digital audio signal may be
formatted according to a standardized serial data communication
protocol such as IIC or SPI, or formatted according to a digital
audio protocol such as I.sup.2S, SPDIF etc.
The present excursion control system may advantageously function as
a self-contained audio delivery system with integral loudspeaker
excursion detection and control that can operate independently of
any particular environment and application processor to provide
reliable and convenient protection against excursion induced
mechanical damage of the electrodynamic loudspeaker.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will be described in more
detail in connection with the appended drawings, in which:
FIG. 1 is a schematic cross-sectional view of a 6.5''
electrodynamic loudspeaker for various sound reproducing
applications suitable for use in the present invention,
FIG. 2 shows an experimentally measured plot of voice coil
inductance versus diaphragm excursion for the 6.5'' electrodynamic
loudspeaker,
FIG. 3 shows measured voice coil impedance versus frequency for the
electrodynamic loudspeaker illustrated on FIG. 1 above,
FIG. 4 is a schematic block diagram of a loudspeaker excursion
detector for electrodynamic loudspeakers in accordance with a first
embodiment of the invention,
FIG. 5A) shows a composite drive signal applied to the voice coil
of the electrodynamic loudspeaker by the loudspeaker excursion
detector of FIG. 3 above,
FIG. 5B) shows a measured filtered voice coil current waveform of
the electrodynamic loudspeaker in response to the application of
composite drive signal illustrated above on FIG. 5A); and
FIG. 6 shows a time-zoomed portion of the filtered voice coil
current waveform displayed on FIG. 5B) above.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 is a schematic illustration of a typical electrodynamic
loudspeaker 100 for use in various types of audio applications. The
skilled person will appreciate that electrodynamic loudspeakers
exist in numerous shapes and sizes dependent on the intended type
of application. The electrodynamic loudspeaker 100 used in the
below described methodologies and devices for loudspeaker excursion
detection and control has a diaphragm diameter, D, of approximately
6.5 inches, but the skilled person will appreciate that the present
invention is applicable to virtually all types of electrodynamic
loudspeakers, in particular to miniature electrodynamic loudspeaker
for sound reproduction in portable terminals such as mobile phones,
smartphones and other portable music playing equipment. The maximum
outer dimension D such miniature electrodynamic loudspeakers may
lie between 6 mm and 30 mm.
The electrodynamic loudspeaker 100 comprises a diaphragm 10
fastened to a voice coil former 20a. A voice coil 20 is wound
around the voice coil former 20a and rigidly attached thereto. The
diaphragm 10 is also mechanically coupled to a speaker frame 22
through a resilient edge or outer suspension 12. An annular
permanent magnet structure 18 generates a magnetic flux which is
conducted through a magnetically permeable structure 16 having a
circular air gap 24 arranged therein. A circular ventilation duct
14 is arranged in a center of the magnetically permeable structure
16. The duct 14 may be used to conduct heat away from an otherwise
sealed chamber situated beneath the diaphragm 10 and dust cap 11. A
flexible inner suspension 13 is also attached to the voice coil
former 20a. The flexible inner suspension 13 serves to align or
center the position of the voice coil 20 in the air gap 24. The
flexible inner suspension 13 and resilient edge suspension 12
cooperate to provide relatively well-defined compliance of the
movable diaphragm assembly (voice coil 20, voice coil former 20a
and diaphragm 10). Each of the flexible inner suspension 13 and
resilient edge suspension 12 may serve to limit maximum excursion
or maximum displacement of the movable diaphragm assembly.
During operation of the loudspeaker 100, a drive signal voltage is
applied to the voice coil 20 of the loudspeaker 100. A
corresponding voice coil current is induced in response leading to
essentially uniform vibratory motion, in a piston range of the
loudspeaker, of the diaphragm assembly in the direction indicated
by the velocity arrow V. Thereby, a corresponding sound pressure is
generated by the loudspeaker 100. The vibratory motion of the voice
coil 20 and diaphragm 10 in response to the flow of voice coil
current is caused by the presence of a radially-oriented magnetic
field in the air gap 24. The applied coil current and voltage lead
to power dissipation in the voice coil 20 which heats the voice
coil during operation. Consequently, prolonged application of too
high drive voltage/current may lead to overheating of the voice
coil which is a common cause of failure or irreversible damage in
electrodynamic speakers. The application of excessively large voice
coil currents which force the movable diaphragm assembly beyond its
maximum allowable excursion limit is another common fault mechanism
in electrodynamic loudspeakers leading to various kinds of
irreversible mechanical damage. One type of mechanical damage may
for example be caused by collision between the lowermost edge of
the voice coil 20 and an annular facing portion 17 of the
magnetically permeable structure 16.
A significant source of non-linearity of the loudspeaker 100 is
caused by the excursion or displacement dependent length of voice
coil wire placed in the magnetic field inside the magnetic gap 24.
From the schematic illustration of the loudspeaker 100 it is
evident that the length of voice coil wire arranged in proximity to
the magnetically permeable structure 16 tends to decrease for large
positive (upwards) excursion and increase for large negative
excursions of the voice coil 20. Due to this variation of the
amount of magnetically permeable material close to the voice coil
with voice coil/diaphragm excursion, the inductance of the voice
coil 20 exhibits a similar excursion dependent variation which is
utilized in the present invention as explained in further detail
below.
FIG. 2 shows an experimentally measured plot 200 of voice coil
inductance, L.sub.e, of the 6.5'' electrodynamic loudspeaker 100
discussed above versus diaphragm excursion. The measured voice coil
inductance is indicated in Henry along the y-axis of the graph 2
and the diaphragm excursion from its quiescent position in mm is
indicated on the x-axis. The quiescent position of the diaphragm
(and hence of voice coil assembly) corresponds to x=0. The
pronounced lack of symmetry in the inductance curve on either side
of the quiescent position is evident. The inductance increases for
negative displacement (inward) and decreases for positive
displacement (outward). This lack of symmetry is caused by the
markedly asymmetric geometry of the magnetic circuit adjacent to
the air gap 24.
FIG. 3 shows a measured impedance curve 305 for the 6.5''
electrodynamic loudspeaker discussed above across a frequency range
from 10 Hz to about 100 kHz. The loudspeaker may produce useful
sound pressure in a certain sub-range such as a frequency range
between about 50 Hz and 10 kHz depending on amongst other factors,
dimensions of the loudspeaker enclosure and shape of the
loudspeaker diaphragm. A DC resistance of the voice coil of the
loudspeaker is approximately 3.5.OMEGA. as evidenced by the
measured 10 Hz impedance. The low-frequency or natural resonance
frequency of the loudspeaker is located approximately at 50 Hz
where the impedance 303 reaches a low-frequency peak value of about
50.OMEGA.. Above the natural resonance frequency of the
loudspeaker, the loudspeaker impedance curve 305 exhibits a
constantly rising impedance which is particularly pronounced for
frequencies above approximately 3 kHz. This rise of impedance is
caused by inductance of the voice coil and continues to frequencies
well above 100 kHz for the loudspeaker under examination. The
vertical arrow 308 illustrates the non-linear
excursion/displacement dependence of the voice coil impedance at
high frequencies caused by the previously explained excursion
dependent change variation of the voice coil inductance L.sub.e.
The influence of the excursion dependent change of the voice coil
inductance on the voice coil impedance becomes particularly
pronounced at high frequencies because the voice coil inductance
L.sub.e tends to dominate the voice coil impedance in this
frequency region.
The vertical arrows 304, 306 illustrate the influence on the
impedance curve 305 of a temperature dependent variation of the DC
resistance of the voice coil. Finally, the horizontal arrow 307
illustrates a temperature and excursion/displacement dependent
variation of the natural resonance frequency of the loudspeaker 100
due to a change in suspension compliance.
The pronounced variation of voice coil impedance with diaphragm
displacement at high frequencies is exploited by the present
invention to detect the excursion of the diaphragm and voice coil
assembly. The variation of the voice coil impedance is measured at
a selected frequency by adding a high-frequency probe tone to the
ordinary audio signal (e.g. speech and/or music) and form a
composite drive signal which is applied to the voice coil of the
loudspeaker through a suitable low output impedance power amplifier
such as an analog or digital class D power amplifier. By detecting
the degree or level of modulation of the probe signal current
flowing through the voice coil in response to the application of
the composite drive signal, it is possible to detect the excursion
of the diaphragm and voice coil as explained in further detail
below
FIG. 4 shows a schematic block diagram of a loudspeaker excursion
detector 300 in accordance with a first embodiment of the invention
coupled to the electrodynamic loudspeaker 100 discussed above
through a pair of externally accessible speaker terminals 411a,
411b. In the present embodiment, the loudspeaker excursion detector
300 operates in the digital domain, but other embodiments may
instead use analog signals or a mixture of analog and digital
signals. The loudspeaker excursion detector 300 comprises an audio
signal input, In, for receipt of a digital audio signal supplied by
a Digital Signal Processor (DSP) 302. Hence, the DSP 302 functions
inter alia as a digital audio signal source of the present
loudspeaker excursion detector 400. The digital audio signal
supplied by the DSP 402 may be generated by the DSP itself or
derived from an external digital audio source, for example a
digital microphone, and supplied to the DSP 402 through the audio
input 401. An externally generated digital audio signal may be
formatted according to a standardized serial data communication
protocol such as IIC or SPI, or formatted according to a digital
audio protocol such as IIS, SPDIF etc. The loudspeaker excursion
detector 400 is supplied with operating power from a positive power
supply voltage V.sub.DD. Ground (not shown) or a negative DC
voltage may form a negative supply voltage for the loudspeaker
excursion detector 400. The DC voltage of V.sub.DD may vary
considerably depending on the particular application of the
loudspeaker excursion detector 400 and may typically be set to a
voltage between 1.5 Volt and 100.0 Volt.
The skilled person will appreciate that the illustrated loudspeaker
excursion detector 400, the DSP 402 and the loudspeaker 100 may
form part of a complete excursion control system for the
electrodynamic loudspeaker 100. In particular, the DSP 402 and
loudspeaker excursion detector 400 may be integrated on a common
semiconductor substrate connectable to the loudspeaker 100 through
the illustrated pair of externally accessible speaker terminals
411a, 411b. The DSP 402 is configured to internally process digital
signals by a sampling frequency of 48 kHz derived from the external
DSP clock input, f_clk1. The external DSP clock input, f_clk1 may
be set to a clock frequency between 10 MHz and 100 MHz. The
sampling frequency may be selected to other frequencies such as a
frequency between 16 kHz and 192 kHz, in other embodiments of the
invention depending on factors like desired audio bandwidth and
other performance characteristics of a particular application. The
digital audio signal supplied by the DSP 402 to the input of the
loudspeaker excursion detector 400 has a sampling frequency of 48
kHz. The loudspeaker excursion detector comprises a probe signal
source (not shown) generating and supplying the previously
discussed high-frequency probe signal in digital format to the
loudspeaker excursion detector 400 through terminal 403. The probe
signal may either by generated by the DSP 402 at the same sample
rate as the digital audio input signal or by an independent digital
probe signal source or generator with another sample rate.
The loudspeaker excursion detector 400 comprises a digital PWM
output amplifier comprising a composite up-sampler and modulator
404 coupled to an H-bridge output stage 406. The H-bridge output
stage supplies the composite drive signal in a pulse width
modulated format to the loudspeaker 100 through the pair of output
terminals 411a, 411b. The digital PWM output amplifier is
configured to exhibit an output impedance, at the pair of output
terminals, that is significantly lower than the impedance of the
driven loudspeaker 100 at the frequency of the digital probe signal
to provide essentially constant voltage drive to the loudspeaker
100 for reasons discussed below in further detail. The output
impedance of the digital PWM output amplifier at the probe signal
frequency may be less than 1.0.OMEGA., even more preferably less
than 0.5.OMEGA., such as less than 0.1.OMEGA..
The loudspeaker excursion detector 400 additionally comprises a
current detector schematically illustrated by the arrow I.sub.sense
407 that detects a composite drive signal current I.sub.L flowing
through the voice coil of the loudspeaker 100 in response to the
application of the composite drive signal by the digital PWM output
amplifier to the loudspeaker 100. The skilled person will
appreciate that the current detector may comprise various types of
current sensors that generate a voltage, current or charge signal
proportional to the composite drive signal current in the voice
coil for example a current mirror connected to an output transistor
of the H-bridge 406 or a small sense resistor coupled in series
with the loudspeaker 100. The composite drive signal current
I.sub.L may accordingly be represented by a proportional/scaled
sense voltage which is applied to the input of the
analog-to-digital converter 408. The analog-to-digital converter
408 is adapted to digitize the measured sense voltage and provide a
digital sense voltage or sense data at a sample rate fixed by the
analog-to-digital converter 408 to a suitable input port of the DSP
402. The resolution of the analog-to-digital converter 408 may vary
depending on how accurate value of the sense voltage has to be
represented. In numerous applications, the resolution may fall
between 8 and 24 bits. In one embodiment, the sampling frequency of
the analog-to-digital converter 408 is set to a frequency at least
two times higher than the frequency of the digital probe signal to
ensure accurate representation thereof without aliasing errors. In
the present embodiment with a probe signal frequency around 40 kHz
this requirement means the sampling frequency of the converter 408
should be larger than 80 kHz for example 96 kHz. However, according
to an alternative embodiment of the invention, the sampling
frequency of the converter 408 is synchronized with the digital
probe signal such that the digital output of converter 408 can be
digitally processed to directly down convert or transpose the
spectral content of the composite drive signal current from the
probe frequency to DC. This direct down conversion leaves the
envelope portion of the composite drive signal current centred
around DC. This embodiment of the present loudspeaker excursion
detector 400 allows the use of a digital lowpass filter instead of
the previously discussed analog or digital band-pass filter to
extract the probe signal current. Another advantage of this
embodiment is that it allows the use of a digital decimation
circuit or stage after the digital lowpass filter to reduce the
sample-rate resulting in lower digital power consumption and lower
MIPs requirements of the DSP 402.
The DSP 402 preferably comprises a software programmable DSP core
controlled by executable program instructions such that each signal
processing function may be implemented by a particular set of
executable program instructions. However, the skilled person will
understand that the DSP 402 in the alternative may be essentially
hard-wired such that each signal processing function is implemented
by a particular collection of appropriately configured
combinatorial and/or sequential logic circuitry.
The DSP 402 comprises a software or custom hardware implemented
modulation detector (not shown) configured to determine the
modulation level of the probe signal current of the composite drive
signal current I.sub.L represented by the proportional digital
sense voltage transmitted V.sub.sense to the input port of the DSP
402. As explained above, the modulation detector is preferably
implemented as a set of executable program instructions. The
detection of the modulation level of the probe signal current is
explained in further detail below in connection with the
illustration of experimentally measured waveforms of the composite
drive signal current I.sub.L in the loudspeaker 100.
As explained above, the digital probe signal is added to the
digital audio signal inside the composite up-sampler and modulator
404, rather than inside the DSP 402, which leads to certain
benefits in many embodiments of the invention. The digital probe
signal has a frequency of about 40 kHz in the present embodiment
due to the particular high-frequency impedance characteristics of
the loudspeaker 100. However, since the DSP 402 uses the previously
discussed internal sampling rate of 48 kHz for representation of
digital audio signals, the frequency of the probe signal lies above
the Nyquist frequency of the DSP 402 making the DSP incapable of
accurately representing and manipulating the digital probe signal.
While one solution to this problem would be to use a higher
sampling rate for the internal digital audio signals of the DSP
402, this solution is undesirable in some embodiments because of
the accompanying increase of power consumption. This problem has
been solved in an advantageous manner in the present embodiment by
adding the digital probe signal to an existing intermediate digital
audio signal at an intermediate sample rate inside the composite
up-sampler and modulator 404. The skilled person will understand
the up-sampler or up-sampling circuit may be configured to increase
the 48 kHz sampling rate of the digital audio signal by a
predetermined integer or non-integer factor, for example a factor
between 4 and 32, by one or more intermediate up-sampling stages to
produce the intermediate digital audio signal. According to a
preferred embodiment of the invention, the digital audio signal is
up-sampled in one or more cascaded stages providing the
intermediate digital audio signals at their respective intermediate
sample rates. In one exemplary embodiment, the up-sampling circuit
is configured for 8:1 up-sampling (factor 8) and comprises of three
cascaded 2:1 up-sampling stages or operations. The digital
high-frequency probe signal may be added at any up-sampling stage
where the intermediate or local sample rate meets the Nyquist
condition for the chosen probe signal frequency. The composite
drive signal is therefore generated inside the composite up-sampler
and modulator 404 by adding the digital probe signal to a selected
intermediate digital audio signal at an intermediate sample rate.
The skilled person will appreciate that various types of audio
signal quantisation and noise shaping of the composite drive signal
may be applied in the modulator portion to form a final pulse width
modulated drive signal applied to the inputs of the H-bridge
406.
Finally, the skilled person will understand that the digital probe
signal may be added to the digital audio signal inside the DSP 402
in alternative embodiments of the invention. This is particularly
of interest if the chosen internal signal sampling rate of the DSP
402 from the onset is more than two times higher than the intended
frequency of the digital high-frequency probe signal or in
situations where an increase of the internal signal sampling rate
to accommodate the digital high-frequency probe signal digital is
acceptable.
The waveform graph 500 of FIG. 5A) shows a composite drive signal
applied to the voice coil of the electrodynamic loudspeaker 100
through the pair of externally accessible speaker terminals 411a,
411b of the loudspeaker excursion detector of FIG. 4 above. The
composite drive signal comprises an alternately small/large 60 Hz
signal component, which simulates a variable level of a
low-frequency audio signal, and a constant amplitude high-frequency
probe signal of 40 kHz. The small level time periods of the 60 Hz
signal leads to low excursion of the movable voice coil assembly
and hence relatively constant value of the voice coil inductance
L.sub.e as explained in connection with FIGS. 2 & 3 above. On
the other hand, the time periods where the 60 Hz component of the
composite drive signal has a high level leads to large excursion of
the movable voice coil assembly and hence relatively large
excursion dependent change of the voice coil inductance L.sub.e as
explained in connection with 2 above.
The output impedance of the loudspeaker excursion detector 500 at
40 kHz is significantly smaller than the 32.OMEGA.@ 40 kHz
impedance of the loudspeaker 100 (refer to the impedance curve 505
depicted on FIG. 3). The 40 kHz output impedance of the loudspeaker
excursion detector 500 may for example lie below 1.0.OMEGA. such
that a substantially constant level of the composite drive signal
drive voltage is applied to the loudspeaker voice coil independent
of the previously described variable high-frequency impedance of
the loudspeaker caused by the excursion dependent change of the
voice coil inductance L.sub.e.
The voltage drive of the voice coil of the loudspeaker at the 40
kHz probe frequency leads to a pronounced variable probe signal
current through the voice coil if the 40 kHz impedance of the voice
coil changes with loudspeaker excursion, i.e. at large excursion of
the movable diaphragm and voice coil assembly as explained above.
Under the opposite condition, at small excursions of the movable
diaphragm and voice coil assembly, the constant voltage drive of
the voice at the 40 kHz probe frequency leads to a substantially
constant probe signal current through the voice coil because the 40
kHz impedance of the voice coil remains largely constant
independent of the loudspeaker excursion.
This phenomenon is illustrated on graph 502 which shows a
band-pass-filtered voice-coil current waveform 505 zoomed in time
around a high level to low level transition of the 60 Hz component
of the audio drive signal. The filtered voice coil current waveform
505 has been obtained by filtering by a band-pass filter centred at
the probe signal frequency of 40 kHz. The depicted filtered voice
coil current waveform evidently displays a high level of
modulation, as indicated by arrow 501 tracking top and bottom of
the envelope of the filtered voice coil current waveform, when the
level of the 60 Hz drive signal component is large, i.e. from t=8.5
s to 8.6 s. The maximum and minimum amplitude of the filtered probe
signal current in this region correspond to the maximum and minimum
values of the 60 Hz input signal. Conversely, a low level
modulation, as indicated by arrow 503, is evident under low level
conditions of the 60 Hz drive signal component from t=8.6 s to 8.85
s. Hence, by detecting the envelope modulation of the filtered
voice coil current waveform, the displacement of the movable
diaphragm assembly can be detected. The skilled person will
appreciate that the actual detection of the modulation level of the
probe signal current may be accomplished in various ways in either
the analog or digital domain for example by traditional AM
demodulation techniques including signal rectification and low-pass
filtering. In other embodiments, the modulation level of the probe
signal current may be detected or estimated by applying suitable
bottom and top trackers to the filtered voice coil current waveform
of graph 502. This may be accomplished in the digital domain by a
suitable software function executed by the DSP 402 (refer to FIG.
4) operating on a digitized version of the probe signal current
waveform supplied by the analogue-to-digital converter 508.
The DSP 402 may in addition to the above outlined detection of the
diaphragm/voice coil excursion in addition be configured to limit
or control the diaphragm excursion. This excursion control may be
accomplished by a variety of mechanisms. In one embodiment a
maximum allowable excursion of the electrodynamic loudspeaker is
determined during a calibration measurement on the electrodynamic
loudspeaker or an electrodynamic loudspeaker of the same type. The
modulation level of the probe signal current corresponding to the
maximum allowable excursion is recorded as a maximum modulation
threshold or similar modulation level criteria. During subsequent
operation of the loudspeaker excursion detector 400, the
instantaneous modulation level of the probe signal current is
compared to the maximum modulation threshold by a suitably
configured software/program routine running on the DSP 402. If the
instantaneous modulation level of the probe signal current exceeds
the maximum modulation threshold, the DSP 402 in response
attenuates the level of the digital audio input signal to the
loudspeaker excursion detector 400 for example by selectively
attenuating low-frequency components of the digital audio input
signal (which are more likely to drive the loudspeaker above its
maximum allowable excursion limit) or broad band attenuating the
entire spectrum of the digital audio input signal.
Finally, the skilled person will understand that the frequency of
the high-frequency probe signal can deviate considerably from the
40 kHz frequency utilised in the present embodiment dependent on
impedance characteristics of the specific electrodynamic
loudspeaker. Furthermore, the frequency of the high-frequency probe
signal should preferably be sufficiently high to render it
inaudible either because the frequency lies above the audible band
of human hearing (i.e. above 20 kHz) or because the loudspeaker is
incapable of reproducing noticeable sound pressure at the probe
signal frequency. The selection of probe signal frequency may
accordingly vary considerably depending on acoustic and electrical
characteristics of the loudspeaker type in question; A large
diameter woofer may produce no sound response above for example 1
kHz such that the high-frequency probe signal may be placed at, or
slightly above, 1 kHz for this type of loudspeaker. A small
diameter full-range miniature electrodynamic loudspeaker for
portable communication devices or music players may on the other
hand produce significant sound pressure up to 15 kHz or even 20 kHz
such that the high-frequency probe signal preferably should be
placed at, or slightly above, 20 kHz for this type of loudspeaker
to remain inaudible. Furthermore, the high-frequency probe signal
is preferably also located in a frequency range where the voice
coil impedance of the loudspeaker exhibits a pronounced inductive
behaviour. This is preferred because the excursion detection
methodology and devices are based on the above described excursion
dependent behaviour of the voice coil inductance L.sub.e.
FIG. 6 shows a time-zoomed simulation of the filtered voice coil
current waveform corresponding to the measured waveform 505 of
graph 502, but for a condition where the movable diaphragm assembly
has been blocked from further excursion for example by mechanical
contact with a magnetic circuit structure of the loudspeaker. With
reference to FIG. 1, this situation corresponds to the discussed
collision between the lowermost edge of the voice coil 20 and the
annular facing portion 17 of the magnetically permeable structure
16. The present inventors have determined that certain features of
the filtered voice coil current waveform are highly useful to
detect that the movable diaphragm assembly of the loudspeaker has
reached or exceeded its maximum allowable excursion, or excursion
limit. Hence, mechanical damage of the voice coil is a likely
result unless precautionary measures are taken to limit the
excursion. The fact that this determination can be made from the
filtered voice coil current waveform itself without any a priori
knowledge of linear and non-linear properties of the loudspeaker in
question is highly useful. This feature may eliminate the need for
individual calibration of the previously discussed excursion
control system to the connected electrodynamic loudspeaker.
The displayed segment of the filtered voice coil current waveform
on graph 600 is centred around a single peak of the envelope of the
filtered voice coil current waveform. The displayed voice coil
current waveform 605 comprises a substantially flat peak plateau as
indicated by the dotted box 607. The simulated change of the voice
coil inductance in percentage is indicated by curve 601 along the
y-axis. Curve 601 also displays a substantially flat peak plateau
as indicated by the dotted box 603. The abrupt stop to the
excursion induced change of the voice coil inductance indicates
that the excursion of the movable diaphragm assembly (thereby also
of the voice coil) has been abruptly stopped in the same manner,
e.g. by collision with the magnetic circuit structure as mentioned
above. The detection of exactly when the movable diaphragm assembly
of the loudspeaker has exceeded its excursion limit can be carried
out by initially identifying these substantially flat peak plateaus
in the voice coil current waveform 605. Thereafter, the shape of
the current waveform 605 can be correlated with the corresponding
waveform shape of the loudspeaker drive voltage, for example
represented by the waveform of the audio input signal. If the
loudspeaker drive voltage does not possess a corresponding flat
peak plateau at the location of the flat peak plateau in the voice
coil current waveform 605, this condition indicates the
above-discussed abrupt arrest of excursion of the movable diaphragm
assembly.
The non-zero portion of the rectangular curve 609 indicates a time
segment of the voice coil current waveform 605 where the movable
diaphragm assembly is estimated to exceed its excursion limit. This
estimate has been computed by applying the above-mentioned
technique based on the detection of correlated flat peak plateaus
of the voice coil current waveform 605 and loudspeaker drive
voltage.
* * * * *